LE3 .A278 2009

2009

Tong, Anthony

Acadia University

Bachelor of Science

Honours

Chemistry

Chemistry

Chemical sensing of environmental samples can be time consuming and expensive as samples must be collected and then transported to the laboratory for analysis. Evanescent-wave fibre-cavity ring-down spectroscopy (FCRDS) is a potential solution to these issues. A linear FCRDS apparatus for evanescent sensing was constructed using 100 μm core multimode optical fibres. Optical fibre ends were coated with dielectric mirrors to form optical cavities and the light intensity was modulated inside the cavities from 10 kHz to 250 kHz using an acousto-optic modulator. Phase-shift measurements were conducted, allowing for the calculation of the ring-down time of the optical cavity. The phase-shift measurements were performed using a tuneable diode laser ranging from 1520 nm to 1570 nm. The ring-down time was determined to be between 438 ns and 819 ns for a 2.3 m cavity, and approximately 441 ns for a 9.0 m cavity. Circular imperfections were observed in the dielectric coating using a scanning electron microscope (SEM). This was believed to have been the cause for the high signal loss and low coupling efficiency of the optical cavities. Evanescent wave experiments were conducted with acetylene and it was determined that the evanescent field was confined within the unmodified optical fibres and was not accessible to the surrounding gas. Future work will involve removing the polymer jacket from the optical fibre and chemically etching the fibre cladding to expose the evanescent field. It is believed that the new device will be useful in many applications in field sensing of greenhouse gases and other emerging environmental contaminants.

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https://scholar.acadiau.ca/islandora/object/theses:620